1. Field of the Invention
The present invention relates to a construction machine provided with a working oil tank such as a hydraulic excavator.
2. Description of the Related Art
THE RELATED ART will be described taking a hydraulic excavator of a preferred example of the present invention as an example.
The hydraulic excavator is, as shown in FIG. 5, formed by rotatably mounting an upper rotating body 2 on a crawler type lower traveling body 1 and attaching a working attachment 3 to the upper rotating body 2.
The working attachment 3 is formed by a boom 4 freely raising and lowering, an arm 5 attached to an front end of the boom 4, a bucket 6 attached to a front end of the arm 5, a boom cylinder 7, an arm cylinder 8 and a bucket cylinder 9 serving as hydraulic actuators for driving the boom, the arm and the bucket.
FIG. 6 shows device arrangement of a part of the upper rotating body 2, and FIG. 7 is an enlarged view of a part of FIG. 6.
An engine 10 serving as a power source and a hydraulic pump 11 serving as a hydraulic source are installed in a rear part of an upper frame 12 serving as a base of the upper rotating body 2. On one of left and right sides (on the right side in an example of the figure) on the front side of the engine 10 and the hydraulic pump 11, are installed and laterally aligned a control valve 13 for controlling an action of the hydraulic actuator such as each of the hydraulic cylinders 7 to 9, and a working oil tank 14 for supplying working oil to the hydraulic actuator, with the control valve being placed on the outer side.
It should be noted that a guard cover 15 is openably attached to an end on the right side of the upper frame taking the front side as center. By opening the guard cover 15, maintenance of the control valve 13 is performed.
The control valve 13 is, as shown in FIG. 7, formed such that a plurality of main spools (one main spool is shown in the figure) 16 for controlling the working direction and speed on the basis of a lever operation for each actuator are built in one valve block 17 in a story shape.
In the control valve 13, the main spools 16 are provided in parallel with the working oil tank 14, that is, in the longitudinal direction, due to tube connection to each port or the like.
When there is a need for a so-called pressure compensating function for compensating an action pressure of the hydraulic actuator and ensuring a fixed operability irrespective of a change in load, a pressure compensating spool (also called as load sensing spool) 18 is provided at right angles with the main spool 16, that is, in the lateral direction.
In FIG. 7, the reference numeral 19 denotes a bolt for closing a take-in and take-out port for the pressure compensating spool 18. At the time of assembling a valve and at the time of maintenance, the pressure compensating spool 18 is attached to and detached from the valve block 17 in a state that the bolt 19 is removed.
Meanwhile, a configuration the working oil tank 14 as THE RELATED ART is shown in FIGS. 8 and 9.
The working oil tank 14 is formed such that a cylindrical filter case 21 having a bottom is attached within a tank main body 20, a return filter 22 is housed inside thereof, and return oil flowing from the hydraulic actuator through a plurality of return tubes 23 is filtered in the return filter 22, and then returned to the inside of the tank main body 20 by a return pipe 24 installed through a central part of a filter case bottom wall 21a. 
The return filter 22 is disposed on the bottom wall 21a of the filter case 21 and held by a push spring 25.
In the figure, the reference numeral 26 denotes a suction filter for filtering feed oil and the reference numeral 27 denotes a suction tube.
In the tank structure, as THE RELATED ART, all the return tubes 23 are connected to the upper side than the return filter 22, that is, to an upper part of a filter case side wall 21b. 
In the above configuration, a point that the control valve 13 and the working oil tank 14 are aligned is shown in Japanese Patent Laid-Open No. Hei11-190449, a control valve structure including the pressure compensating spool 18 is shown in Japanese Patent Laid-Open No. 2002-13656, and a configuration of the working oil tank 14 is shown in Japanese Patent Laid-Open No. 2005-282805.
When taking a device layout shown in FIGS. 6 and 7, at the time of maintenance of the control valve 13, the main spool 16 is easily attached and detached in the longitudinal direction due to spaces on both the front side and the rear side. However, since a left side space S which is on the working side for performing attachment and detachment of the pressure compensating spool 18 is closed by the working oil tank 14, it is difficult or impossible to perform an attachment and detachment work of the pressure compensating spool 18.
Therefore, at the time of maintenance, the entire control valve 13 has to be removed from the upper frame 12, and hence the work is very troublesome.
As countermeasures thereof, it is thought that (A) the working oil tank 14 is displaced to the inner side (the left side) and a sufficient working space is ensured between the working oil tank 14 and the control valve 13, and (B) the working oil tank 14 is downsized in the lateral direction.
However, it is difficult to realize the countermeasure (A) since in the hydraulic excavator, particularly a small excavator of small rotating type or the like, a device installation space is limited or the like.
It is also difficult to realize the countermeasure (B), since a tank capacity of the working oil tank 14 is decreased so that it is not possible to ensure a necessary amount of working oil.
It should be noted that a return structure of the filter case 21 or the like shown in FIGS. 8 and 9 is downsized (reduced in diameter) so that an effective capacity of the tank main body 20 is increased and hence the tank is downsized.
However, in the working oil tank 14 mentioned above, all the return tubes 23 are concentrated on and connected to the upper part of the side wall 21 of the filter case in an octopus shape. Therefore, in order to ensure a tube connection area, a perimeter of the filter case 21 has to be large. As a result, it is substantially impossible to reduce the diameter of the filter case 21, and hence it is actually difficult to downsize the tank.